The casting mold parts of the type described above typically involve in practice what are referred to as “casting cores”, with which cavities such as channels, hollows, etc., or cut-out apertures with undercuts and comparable complex shapes are formed on the part to be cast. When the finished hardened cast part is removed from the individual casting mold, the casting mold parts concerned are destroyed. In this situation they decompose into fragmentary individual pieces, which can be conveyed out of the cast part mechanically, for example by vibration, or with the aid of a flushing fluid.
Casting cores of this type are used both in casting molds, of which the outer parts are designed as fixed permanent molds, as well as in what are referred to as “lost casting molds”. With lost casting molds, not only the casting cores but also the outer mold parts, surrounding the cast part on the outside, are made of mold material and are accordingly likewise totally destroyed when the individual cast part is removed from the mold.
There are various possibilities known for producing lost mold parts (casting cores and outer mold parts) for casting molds. In this context, a distinction is made between what are referred to “cold-box processes” and “hot-box processes”. While the hot-box processes are based on the use of mold materials containing an inorganic binder, the cold-box processes have the common factor that the mold material, mixed from mold sand and an organic binder, is gassed with a gas after being filled into the mold box forming the casting to be produced in each case. The gas passing through the mold material in this situation reacts chemically with the respective binder and so causes it to harden.
One variant of the cold-box process is the SO2 process. With this process, the mold material being processed in each case is mixed from mold sand and a resin binder, which may be, for example, a furan-phenol or epoxy resin binder. During the gassing of a mold material composed in this manner with SO2, the resin binder hardens due to reaction with the sulphuric acid which forms from sulphur dioxide, oxygen, and water.
The SO2 process is used to a great extent in practice, since mold materials which can be solidified with sulphur dioxide have good flowing properties in the non-solidified state, and therefore inherently have particularly good mold filling capacities. These mold materials are therefore well-suited in particular for the production of filigree-shaped outer parts and cores for casting molds. In addition, the mold materials which can be solidified with sulphur dioxide can be kept for long periods without any special precautions and after gassing with the sulphur dioxide gas have a high degree of mold stability.
Practical experience in the casting of cast iron in casting molds produced in the SO2 process has shown, however, that the cast parts obtained in this situation frequently have undesirable degeneration of the graphite formed in the cast part obtained in this way. This observation related in particular to castings which were cast from an iron melt treated with magnesium.
As described in detail, for example, in EP 1 752 552 B1, cast iron can itself undergo a magnesium treatment immediately before entering the casting mold or while still in the casting mold. The magnesium introduced in this process forms compounds with other constituents of the cast iron or with elements likewise additionally introduced, which serve as nuclei for the formation of the graphite type desired in each case. Accordingly, by suitable additions of magnesium, optimized casting results can be achieved in the production of spheroidal graphite (“GJS”), in which the graphite is present in a spheroidal form, or vermicular graphite (“GJV”), in which the graphite is present in a worm-like shape.
Cast iron with spheroidal graphite has typical strength values from 350 MPa to 1000 MPa, while the strength of cast iron with vermicular graphite lies in the range from 350 MPa to 500 MPa. The particular advantage of vermicular graphite in this situation lies in a favorable combination of high strength and good thermal conductivity, as well as good damping behavior. Cast iron with lamellar-shaped graphite (“GJL”), by contrast, has strength values in the range from 150 MPa to 350 MPa.
It has been observed on cast parts from magnesium-treated cast iron melts manufactured from GJS or GJV in casting molds with SO2-hardened outer parts or casting cores that the graphite in locally delimited sections close to the surface was not present in the expected spheroidal or vermicular form but in lamellar form. This deviation from the formation of graphite actually being striven for leads to locally sharply deviating properties of the cast part, as a result of which the quality of thin-walled parts can be severely impaired.